All categories of waste have created gigantic service industries directed to clean-up and disposal of wastes. One of the large waste problems has been created by the pulp and paper industry, where waste materials have been stored in "sludge," "polishing," "processing," and the like ponds. Such ponds are normally located in the waste system from the mill clarifiers. As the waste gathers in the mill's waste system, the wastes are pumped into clarifier(s) for the start of the separation of solids from liquids.
In the pulp and paper industry, much of the solids material is paper waste becoming fiber in the waste system and accounts for 90%-plus of the wasted solids. As the clarifier separates the solids, they are pumped out of the bottom of the clarifier as they sink and are pumped into these "ponds" for breakdown by microorganisms. Generally, the solids are left in the pond and, through "weir gates," the liquid is returned to the clarifier for further clarifying and, eventually, to an oxidation pond for additional clarification and eventual return to public waterways.
The solid materials left in the pond to further decay and be completely broken down to inert material and dead "bugs" are the subject of our further discussion here. This material is left to accumulate for a number of years and, in time, it fills the total capacity of the pond to a point that the pond is no longer functional and becomes inefficient, sending more and more solids back into the clarifier. These ponds, full of these soft-solids and water, and each covering 80 acres or more, may be 5 to 20 feet deep, and 200 to 800 feet in width. The removal of this soft-solid material from the lagoon and into a retention or land-fill area becomes a real challenge to man and machine.
The state-of-the-art approach for cleaning ponds seems to be as follows:
A. Divert the material "slurry" flow into an alternate pond or divide the pond with a dirt dike, which will later be removed, and divert the flow into one side of the pond. Pump all liquid into the same pond to which the flow of material is being diverted. PA1 B. Reach as far out as possible with a dragline or long-reach excavator to remove all reachable materials from the bank of the pond. PA1 C. Place a light-weight, low ground pressure (LGP) bulldozer into the pit of the pond to push the remaining solids to the bank so that it can be loaded out by conventional means.
Item C. above has many drawbacks; with the soft bottom of the pond and the ooze factor of the sludge, the bulldozer units are bogged down in the bottom much of the time, and these small tractors can push only small payloads, resulting in very low production.
The present invention is not limited to ponds containing soft pulp and liquids and may be useful, as disclosed in U.S. Pat. No. 4,753,737. In the pulp and paper industry, a very caustic solution called white liquor is used in the digesters for cooking wood chips for the production of pulp. This combination (wood chips and white liquor) is sealed in a cooker, and steam is introduced, and it "cooks" under high pressure. After cooking, this mixture is "blown" from the digester and goes through a separation process. This process separates the pulp from the liquid, which is referred to as black liquor. The black liquor contains several byproducts--turpentine and "soap" (from which tall oil is made) are two--plus residual chemicals and B.T.U. (heat) values. After the by-products are removed, the black liquor contains chemical and B.T.U. solids in the magnitude of about 10%. The higher the solids, the greater the value of the black liquor. In order to recover the values, the black liquor is processed through evaporators in which the water is removed, and, in several stages, the solids content is raised to 55%-62% for burning in recovery boilers. In the burning process in the recovery boiler, the heat value is removed for the production of energy, and the chemical elements form a smelt matter called green liquor. This green liquor drops into a heat sink-pool at the bottom of the boiler and is removed for processing through a lime kiln for the production of white liquor. Quite frequently, prior to recovery of the values in the black liquor, the black liquor is stored in ponds, pools and lagoons.
These large lagoons contain vast amounts of liquor, ranging in solids content from weak 2%-20% to heavy 25%50%. They may remain in such a lagoon for months (in some cases, actually years) before limited recovery capacity can reclaim them back into the system. As the black liquor settles, the solids drop to the bottom of the pond as they fall out of solution and suspension. Also, the heavier the solids, the faster they fall. This is especially true as the high processing temperatures fall in the lagoon environment. The black liquor solids form heavy, thick layers over time and become quite firm. The present invention comprises an apparatus for recovery of the black liquor solids from storage ponds.